1. Field of the Invention
This invention relates to an ink jet recording method and apparatus for recording an image on a recording material by ejecting ink droplets in conformity to the data of the image.
2. Related Background Art
In consequence of the dissemination of reproducing devices, information processors such as word processors and computers, and communication devices as well, digital image recording devices adapted to operate by use of an ink jet type recording head have been finding widespread acceptance among other image recording devices serving the apparatuses mentioned above. As a recording head having a plurality of recording elements arrayed integrally therein (hereinafter referred to as a "multi-head"), the recording devices of this kind are generally provided for the sake of enhancing their recording speed with a modified multi-head which has a plurality of ink nozzles and conduits integrally arrayed therein. Further, to permit production of color images, they are provided with a plurality of such multi-heads.
Unlike the monochromic printer which prints characters exclusively, the color printer needs to fulfill various factors such as color development property, gradient of tone, and uniformity in printing color images. As regards the uniformity in particular, even slight inconstancy possibly caused among nozzle units by a deviation involved in the process of manufacture of a multi-head affects the amounts of ink droplets discharged through individual nozzles and the directions in which the ink droplets are ejected in the course of printing and eventually impairs uniform density of a printed image and deteriorates the quality of the produced image.
A concrete example of this adverse case will be depicted below with reference to FIG. 48A to FIG. 48C. In FIG. 48A, 91 stands for a multi-head which is identical with a multi-head shown in FIG. 49A. Here, for the sake of simplicity, this multi-head is assumed to comprise 8 multinozzles 92. Denoted by 93 are ink droplets which are ejected by the multinozzles 92. Generally, the ink is ideally ejected in droplets of a uniform amount in parallel directions as illustrated in the diagram. If the ink is ejected as just described, then ink dots of a uniform size will land on the paper surface as illustrated in FIG. 48B and form a wholly uniform image free from uneven density (FIG. 48C).
Actually, as pointed out previously, the individual nozzles betray inconstancy of quality among themselves. If they are used for printing in spite of the defect, the ink droplets ejected therethrough will vary in size and direction as illustrated in FIG. 49A and land on the paper surface as illustrated in FIG. 49B. From this diagram, it is clearly noted that blank areas unfulfilling the area factor of 100% occur periodically relative to the main scanning direction of the head, dots overlap one another to an unduly large extent on the contrary, or white streaks occur as found in the central part of the diagram. The aggregate of the dots which have landed in this condition describes a density distribution shown in FIG. 49C relative to the direction in which the nozzles are arrayed. As a result, these phenomena are generally perceived as uneven density by the human eyes.
The following method has been proposed as a measure to this uneven density. The method will be described below with reference to FIG. 50A to FIG. 51C. This method requires the multi-head 91 to make three passes (or three scans) to complete such a print area as shown in FIG. 48A to FIG. 48C. One half of the area consisting of four picture element units is completed by two passes of the multi-head 91. In this case, the eight nozzles of the multi-head are divided into two groups, namely the upper and the lower group of four nozzles. The dots printed by one nozzle in one pass (or one scan) are such as result from thinning relevant image data roughly to one half in accordance with a prescribed image data array. Then, in the second scan dots corresponding to the remaining half image data filled or recorded, whereby the printing of the area of four picture element units is completed. The recording method just described will be hereinafter referred to as the "split recording method." This split recording method halves the effects inherently exerted by the individual nozzles on a printed image even when the recording head used therein happens to be equal to the recording head shown in FIG. 49A. The printed image consequently obtained is as shown in FIG. 50B. In this printed image, such black streaks or white streaks as are found in FIG. 49B are not very conspicuous. The uneven density of image, as shown in FIG. 50C, is appreciably alleviated as compared with what is shown in FIG. 49C.
In the recording of image thus performed, the first and the second scans split the image data in a mutually offsetting manner in accordance with a prescribed array. Heretofore, as the image data array (thinning pattern), it has been most popular to adopt such an image data array as to produce a pattern resembling a shepherd's check by each set of one longitudinal and one lateral picture element as illustrated in FIG. 15. In the unit printing area (composed of four picture element units), therefore, the printing is completed by the first scan which prints a shepherd's check and the second scan which prints an inverted shepherd's check. FIGS. 51A, 51B, and 51C portray how the record in a given area is completed with these shepherd's checks and inverted shepherd's checks by use of a multi-head provided with eight nozzles as illustrated in FIGS. 50A to 50C. First, the first scan records a shepherd's check .circle-solid. by use of the four lower nozzles (FIG. 51A). Then, the second scan records an inverted shepherd's check .largecircle. by feeding the paper across four picture elements (one half of the head length) (FIG. 51B). Further, the third scan again records a shepherd's check by feeding the paper across four picture elements (one half of the head length) (FIG. 51C). The record area of four picture element units is completed for each scan by sequentially alternating the feeding of the paper across four picture element units and the recording of a shepherd's check and an inverted shepherd's check as described above. Since the print is completed in one and the same area by use of two different kinds of nozzles as described above, this method permits production of an image of high quality free from uneven density.
The drawing depicts the split recording method as adapted to complete a record in one and the same area by two passes. It should be remarked, however, that the effect of the split recording method gains in conspicuity in proportion as the number of groups into which the nozzles are divided increases. Even the recording apparatus described above is enabled to complete an image in one scanning direction by use of four kinds of nozzles when the number of picture elements to be recorded by one pass is further halved and the scan for paper feeding is given a scanning width of two picture elements (one quarter of the head length). Thus, this apparatus is capable of producing a smoother and more desirable image.
The split recording of this principle, however, has the disadvantage that the time cost for printing a given image on one paper surface increases and the throughput consequently decreases inevitably in proportion as the number of groups into which the nozzles are divided increases. For the purpose of further expediting the printing operation in this case, a method of adapting a carriage to produce printing and scanning operations on both the forward and backward directions may be conceived. In fact, this method can substantially halve the recording time spent normally heretofore on one paper surface because it wholly eliminates the motion which the carriage would have otherwise produced in returning idly to its home position after the recording by one pass has been completed. Actually, the monochromic printing apparatuses which adopt the principle of reciprocating printing are not few.
FIGS. 52A to 52D illustrate a head in the process of moving at a fixed velocity V in the forward or the backward direction while causing an ink drop to be ejected at a fixed velocity v to a smooth paper surface. When the paper surface is flat and smooth as shown in FIG. 52A and, therefore, the distance d between the paper surface and the head face is always constant, the timing of the reciprocating printing motions of the head with respect to the discharge of ink is so set in advance that the position of the dot printed on the forward pass may coincide with that of the dot printed on the backward pass. When the paper surface is somehow caused to rise from the normal level as shown in FIG. 52C and, consequently, the distance d between the head face and the paper surface becomes shorter (d') and the interval between the time the ink drop departs from the head and the time it reaches the paper surface differs from the actually preset interval in each of the forward and the backward directions, the positions of the printed dots deviate from the target position as shown in FIG. 52D.
Now, the detriment which arises when a thinning mask is used in the condition described above and the production of a 100% duty image is attempted by the reciprocating printing operation will be described below. FIGS. 2A and 2B illustrate examples of the picture element array obtained by a varying recording scan in accordance with the split recording method designed to complete an image by four recording scans and the condition of a print of dots obtained therein. The picture element array to be printed by each recording scan has thinning masks (a) to (d) for the picture element arrays which are complementary to one another. In this case, the picture element array of the thinning mask used for the first recording scan and that used for the third recording scan are printed during the forward pass of the head, while those used for the second and the fourth recording scan are printed during the backward pass of the head. In the diagram, the number of picture elements recorded in the forward pass and that recorded in the backward pass are equal. If the deviation of dots shown in FIGS. 52C and 52D occurs under this condition, the dots printed during the forward pass and those printed during the backward pass will deviate from each other and give rise to gaps between the horizontal rows of dots and between the vertical columns of dots to the extent of imparting a coarse appearance conspicuously to the produced image as shown in FIG. 2B. The image produced in this state betrays uneven density and inferior linearity of characters and lines because of the uneven arrangement of dots.
As described thus far, an attempt to realize the split recording and the reciprocating printing simultaneously for the purpose of exalting the quality of image and expediting the printing operation results in yielding a printed image impaired by the deviation of dot positions which occurs during the reciprocating printing described above.
The split recording method of this principle has been already disclosed as in JP-A-60-107,975 and U.S. Pat. No. 4,967,203. It is described as profoundly effective in countering such adverse phenomena as uneven density and lengthy streaks. The former patent specification defines this method as "being characterized by comprising means to assign a smaller width to the area of paper feed by each main scan than the width of said main scan and impart an overlapping part to the widths of two adjacent main scans and means to array printed dots in said overlapping part in such a manner as to prevent said printed dots from overlapping each other during said two main scans". According to the method of this specification, the thinning masks are so adapted as to "print an odd-number stage and an even-number stage alternately every other row", or to print an odd-number stage by the first main scan and an even-number stage by the second main scan, or alternatively to produce random recording by each pass. The thinning masks and the paper feed widths are not completely defined.
In contrast, the latter US patent specification (U.S. Pat. No. 4,967,203) discloses this method as "comprising the steps of a) printing by the first pass the non-adjoining but alternating pixel positions in the horizontal and vertical directions only in the upper half part of the first band, b) printing by the second pass the pixels which have escaped being printed by said first pass in said first band and the non-adjoining but alternating pixels in the horizontal and vertical directions in the lower half part of said first band, and c) printing by the third pass the pixels in said first band which have escaped being printed by said first and second passes and, at the same time, effecting a first pass in the immediately following band". Thus, the invention of this US patent specification defines the thinning masks for effecting the split recording as non-adjoining but alternating pixels arranged in the vertical and horizontal directions.
As a supplemental element, the invention under discussion discloses a recording method which comprises forming a pseudo pixel (superpixel) with an aggregate of several picture elements for the sake of gradient expression or multicolor expression and producing non-adjoining but alternating thinned prints with the superpixel units in the horizontal and vertical directions. Regarding this method, the specification has this passage: "Once the system for embodying this method is incorporated in a program software or a printer formware, the program of the system can be retrieved with a combined color number designated with respect to relevant superpixels and, therefore, the quality of print in question can be accomplished without indiscriminately complicating the work of forming a computer program for the production of a host of colors." The simplification of programming for the multicolor expression is adduced as one of the effects of this invention. Further, a mention is made to the effect that since the individual superpixels are intended to be perceived as unique uniform colors, the color bleeding possibly occurring in the superpixels is harmless.
Incidentally, the split recording described above is at a disadvantage in requiring a large time cost for printing on one paper surface and entailing an inevitable decrease in the throughput. For the purpose of further curtailing the time spent in printing in this case, a method of adapting the carrier to produce reciprocating printing and scanning may be conceived. This method, in fact, can substantially halve the time required for recording on one paper surface because it wholly eliminates the motion which the carriage would have made otherwise in idly returning to its home position after the recording by one pass has been completed. Actually, the monochromic printing apparatuses which adopt the principle of reciprocating printing are not few. A color ink jet apparatus constructed as contemplated by the present invention had not yet been realized for the following reason.
FIGS. 54A to 54D are cross sections illustrating drops of the recording inks of popular use today in the process of falling onto a paper surface and subsequently diffusing in the wall of the paper. The diagrams represent the case of causing drops (dots) of the two inks different in color to fall with a time lag at two virtually adjoining positions on the paper surface and then diffuse (recording) in the paper. What should be remarked in this case is the fact that, in the part of the paper at which the two dots have overlapped each other, the dot which has landed on the paper later tends to sink farther in the direction of thickness of the paper than the dot which has landed on the paper earlier. This phenomenon may be logically explained by postulating that during the physical and chemical union of the coloring matter like a dye in the ejected ink with the recording medium, since the union of the recording medium with the coloring matter has its own limit, the union of the coloring matter of the earlier ejected ink with the recording medium proceeds preferentially and, therefore, the dot of this ink remains much on the surface of the recording paper and the coloring matter of the ink landing the paper later is not easily bound on the surface of the recording medium but left sinking farther in the direction of thickness of the paper and lodged fast in the depth of the paper unless the strength of union widely varies with the kind of coloring matter.
If the two inks different in kind are printed at one and the same position in this case, the preference of coloring is varied by the sequence in which the two inks land on the paper. As a result, they will have expressed two different colors to the visual characteristics of man. It is now assumed that the colors of a four-color head are sequentially arranged from right in the order of black, cyan, magenta, and yellow and the head is reciprocated in the direction of arrangement of these colors (left to right) to effect a main scan. In the forward pass (or forward scan), the head is moved to the right and simultaneously caused to perform a recording action. At this time, the order of recording colors conforms to the aforementioned order in which the colors are arranged. When a green (cyan +yellow) signal is injected in a certain area, for example, cyan and yellow inks are absorbed in each of the affected picture elements in the order mentioned. In the present scan, therefore, the earlier absorbed cyan forms a preferential color by the mechanism of union described above and imparts a chromatic taste strongly of cyan to the eventually produced green dot. Conversely, in the backward pass (or backward scan) which follows the paper feeding made in the direction of a secondary scan, the head is moved in the direction opposite to that of the forward pass and simultaneously caused to effect a recording action. As a result, the order of injection of the color inks is reversed. In the present scan, a chromatic taste strongly of yellow is imparted to the produced green dot. When the scanning operation described above is repeated, green dots having a chromatic taste strongly of cyan and green dots having a chromatic taste strongly of yellow are recorded in accordance with forward passes and backward passes which are made by each of the recording heads. If the paper feeding is made by the width of the head as a unit for each of the forward and backward passes without using the split recording method in each pass, the areas of green having a chromatic taste strongly of cyan and those of green having a chromatic taste strongly of yellow will be alternated repeatedly by the width of the head as a unit and, as the result, the eventually produced green image which ought to be uniform in color will raise to serious degradation of quality.
This drawback, however, can be overcome more or less by adopting the conventional split recording method demonstrated hereinabove. Specifically, in the operation of split printing, green dots having a chromatic taste strongly of cyan are recorded by the forward pass (FIGS. 51A and 51C) and green dots having a chromatic taste strongly of yellow are recorded by the backward pass (FIGS. 51B) as already explained with reference to FIGS. 51A to 51C. The overall chromatic taste in a stated area, therefore, is moderated by the intermixture of the dots of the two chromatic tastes.
The construction and effect of moderating the uneven coloration within the individual bands by intermixing the dots of two chromatic tastes within the stated area as described above have been already disclosed in U.S. Pat. No. 4,748,453. Though the US patent specification does not specify any limit for the amount of paper feeding, it has a mention to the effect that the method of this invention has the effect of preventing the beading of ink on the medium such as of an OHP grade paper by causing the first and the second (or more) split recording scan to perform a complementary recording on the picture elements alternately situated in the horizontal and vertical direction in one and the same area and, at the same time, preventing the color banding (uneven coloration) during the formation of a color image by reversing the order of ink injection into picture elements of a mixed color in the first pass (or first scan) and the second pass (or second scan) (reciprocating recording). Since the invention under discussion primarily aims to prevent the beading between the adjacent picture elements, it is characterized by the fact the picture elements which are recorded by one pass (or one scan) are alternated (not mutually adjoined) in the horizontal and the vertical direction.
JP-A-58-194,541 which has issued to the same applicant as the present invention discloses a technique for the operation of an apparatus which is provided with a plurality of parallelly arranged series of recording elements and adapted to effect the main scan of a record of a dot matrix by reciprocating the head in a direction perpendicularly intersecting the series of recording elements mentioned above. This technique, in the operation mentioned above, comprises causing a smaller number of dots than the total number of dots destined to be recorded in at least either of the columns and rows of the dot matrix to be recorded intermittently in the forward pass of the main scan and, at the same time, causing the remaining dots in at least either of the rows and columns of the matrix to be intermittently recorded in the backward pass of the main scan thereby varying the order of overlapping of record in the overlapped recorded dots produced by the aforementioned plurality of series of recording elements in the forward and the backward pass of the main scan. The invention under discussion, unlike the split recording already described, has no restriction designed to decrease the number of rounds of paper feeding from the ordinary number and, as a result, succeeds in preventing a recorded image from being degraded in quality by the deviation of color tone (uneven coloration) due to the overlapped recording of color inks.
Since this invention primarily aims to prevent deviation of color tone, it specifies no specially limited positions for the dots to be recorded by each pass. In the working examples of the invention cited in the patent specification, the lateral thinning for effecting alternate recording only in the longitudinal direction and the longitudinal thinning for effecting alternate recording only in the lateral direction are mentioned in addition to the recording in a checkerwise pattern (a shepherd's check and an inverted shepherd's check).
Besides, JP-A-55-113,573 discloses a construction for effecting a reciprocating recording by use of a twill line (a shepherd's check and an inverted shepherd's check) pattern, though not limited to a color printer. The invention in this case aims to prevent the phenomenon of distortion of dots by avoiding continuous printing of adjacent dots and allowing an immediately succeeding dot to be printed before the immediately preceding dot dried up. Like the invention of U.S. Pat. No. 4,748,453 mentioned above, therefore, the invention under discussion limits the thinning masks to the twill line pattern.
Incidentally, the inventions of the three patents cited above invariably aim to prevent the uneven coloration or beading in the course of reciprocating recording. They, therefore, avoid adopting the construction for "decreasing the amount of paper feeding between adjacent passes to below the ordinary width of heat" for the purpose of preventing the uneven density due to the inconstancy of nozzles in quality unlike the split recording method demonstrated hereinabove.
It has been held that the reciprocating multicolor recording is feasible in spite of being susceptible of uneven coloration because the adoption of the split recording method for the reciprocating recording enables two kinds of recording picture elements for which the order of injection of ink colors is mutually reversed to be uniformly imparted in the recording area.
The drawback of uneven coloration is not thoroughly overcome even when the split recording is effected in the pattern of the shepherd's check/inverted shepherd's check mentioned above. The reason for this failure follows. Generally, the amount of an ink droplet is so designed that it may spread on the paper surface to an area larger than the area assigned to a picture element. The larger area is necessary for wholly concealing the part of white paper surface in the area of data of printing ratio of 100% Even when the split recording method is followed, therefore, the printing medium (recording paper) has practically 100% of the area thereof covered notwithstanding only 50% of the recording picture elements are printed as illustrated in FIG. 53. A cross section taken through this recording paper is illustrated in FIG. 55A to FIG. 55C. The diagram represents the case of using a first pass (in the forward scan) to print a shepherd's check on a white paper and a second pass (in the backward scan) an inverted shepherd's check thereon. The reference numeral 2001 denotes the state of the ink droplet immediately after the printing by the first pass (forward). The part completely filled with black represents cyan ink and the hatched part yellow ink. Since the yellow ink has been injected at the same position as previously occupied by the cyan ink with only a small time lag, the cyan ink is absorbed by the paper in a state of high density with a sign of sparing bleeding and the yellow ink is induced to bleed heavily to the extent of enveloping the lower side and the peripheral part of the cyan ink and eventually assuming a print of low density. Further at this time, these inks are absorbed and spread out so widely as to reach the immediately next picture element, with the result that the entire paper surface will be filled up with the inks as illustrated in FIG. 53.
The print made by the second pass (backward) under the condition mentioned above is superposed on the previously absorbed adjacent dots of ink as indicated by 2003. Since the second pass forms a backward scan, the yellow is printed first and the cyan next (2002). When these two inks are left to be absorbed, they eventually assume a state in which they do not appear very conspicuously to the surface as indicated by 2003. In the printed image finally produced, therefore, the density of the first printed cyan is emphasized most strongly and the area of this print forms a green image having a chromatic taste preferentially of cyan. Conversely, in the area of print adjoining the aforementioned area of print which has used the first pass for the backward scan, the cyan and the yellow change their positions and produce a green image having a chromatic taste preferentially of yellow.
FIG. 56 depicts the manner in which the two areas of print mentioned above appear. It is clearly noted from this diagram that the lower half part of the head always determines the preferential color in each area and this preferential color is reversed in the forward and backward scan. Since these two areas are different in which preferential colors are alternately present, the phenomenon of uneven coloration still persists in the operation of split printing and impairs the produced image and renders the reciprocating printing virtually infeasible.
Further, the drawback caused by the bleeding of ink in the adjoining picture element is found not only the uneven coloration mentioned above but also in the reciprocating monochromic printing. Now, the trouble caused in this case will be described below. FIGS. 57A to 57D illustrate the conditions of ink absorption during a first and a second pass similarly to FIGS. 55A to 55C. Likewise in the diagrams, 2101 denotes the condition of ink which has landed on the paper surface by a first pass and 2102 and 2103 both denote cross sections of the paper which are assumed after the printing by a second pass. Here, 2102 represents the state in which the record by the second pass is formed immediately after the record produced by the first pass and 2103 the state in which the record by the second pass is formed after an interval of some length following the formation of the record by the first pass. These two states show a difference in the state of absorption in the paper surface of the ink recorded by the second pass. While the ink dots 2102 are absorbed fairly in the direction of depth of the paper, the ink dots 2103 produced by the second pass are spread out on the surface of the paper. These behaviors of ink dots are discerned on the reverse side of the paper and the ink dots 2103 permeate to the reverse side of the paper to a greater extent than the ink dots 2102. These states of ink dots are manifested as a difference in density of the two inks on the paper surface as shown in FIG. 57C (2104) and FIG. 57D (2105).
The time lag arising from the reciprocation of the carriage is ample as compared with the order of the time lag responsible for the difference in density between the two kinds of ink dots. This factor constitutes itself as a new drawback attendant on the operation of reciprocating printing. The situation of this drawback will be described below with reference to FIG. 58.
As shown in FIG. 58, first the head makes a forward pass from the position of 2201 in the direction of the arrow to effect a record of the first scan width. After the record of one full line has been completed, the paper is fed by one half of the scan width mentioned above and then the head makes a backward scan from the position of 2202 this time in the opposite direction. Again the paper is fed by the same width as mentioned above and the head then makes a forward scan from the position of 2203 to effect a record in the direction of the arrow. The recording intervals of the two passes will be compared below with respect to the parts (1) to (6) in the area of print completed in this case. In the parts (3) and (4), the record by the second pass is commenced immediately after the record by the first pass has been completed and then the paper has been fed by the one half width. In contrast, in the parts (1) and (6), the record by the second pass is commenced after the carriage, subsequent to the record by the first pass, has completed one reciprocating scan. The parts (2) and (5) are recorded with a time lag exactly one half of the duration intervening between the first and the second record. As a result, as shown in FIGS. 57A to 57D, the parts (1) and (6) acquire the highest density, followed by the parts (2) and (5), and the parts (3) and (4) absorb inks to a great depth in the paper and acquire a low surface density. Thus, the phenomenon of uneven image density appear in the left-hand area in which the passes (1) and (4) by one half width are repeated in the vertical direction and in the right-hand area in which the passes (3) and (6) are repeated. The uneven density impaired the produced image.
The bleeding of the ink of the print by the first pass into the non-printed picture elements is responsible for the fact that the density depends on the recording interval between the first and the second pass. This situation logically explains why the reciprocating printing has not been materialized. The explanation given thus far has assumed the case of applying the reciprocating printing to a monochromic system. The phenomenon in question manifests itself in conjunction with that of uneven coloration even in the case of multicolor recording as already pointed out. In this case, this phenomenon is recognized as prominence of uneven coloration on the left-hand and the right-hand areas or as difference in chromatic taste.
Also in the unidirectional recording, the following detrimental factors affect the time lag of recording. The carriage is temporarily suspended when the recording apparatus performs a head recovery scan for the sake of maintaining its own drive in the course of recording or it keeps itself waiting for arrival of record data being transmitted. Then, the suspension of this nature induces irregular occurrence of uneven image density on a still larger order than the inconstancy of time lag described above. To be specific, the carriage enters the phase of suspension as held in the state ensuing from the production of the record by the first pass and, with a certain time lag, the printed area of recording assumes a higher density than the other areas. This phenomenon of uneven image density induced by the factor mentioned above will be hereinafter referred to as "uneven density due to suspension" for the sake of distinction from the uneven density due to time lag described above.
In the ink jet recording apparatus which forms an image by driving the recording head in a direction different from the direction in which the nozzles in one head are arranged, an effort to realize the split recording and the reciprocating printing with a view to exalting the quality of image and expediting the printing operation still encounter such image drawbacks as uneven coloration, uneven density due to suspension, and uneven density due to time lag.
Further, the reciprocating printing has the possibility of causing positional deviation of ink dots on the paper surface in the forward and the backward printing owing to the accidental rise of the paper from its normal level as pointed out above.
When a 100% duty image is subjected to the reciprocating printing by use of the before-mentioned thinning masks incorporating therein ink dot arrays of the patterns of shepherd's check and inverted shepherd's check, the ink dots land on the paper surface in the manner illustrated in FIG. 58. FIG. 59 depicts the case of performing the split recording by the reciprocating printing using the conventional thinning mask of the pattern of a shepherd's check. The diagram shows the ink dots deviating from their normal positions by one quarter of the size of a picture element. The portions in which adjacent ink dots overlap excessively one another and the portions in which wide gaps intervene between adjacent ink dots are made to appear at different positions owing to the use of thinning masks. In the case of FIG. 59, since all the ink dots are printed in a reverse direction relative to the adjacent ink dots, a gap of the size of one ink dot occurs after each ink dot. Thus, the produced image assumes a low density throughout the entire area thereof.
The positional deviation of ink dots on the paper surface during the reciprocating printing is caused not only by partial rise or fall of the paper surface illustrated in FIGS. 52C and 52D but also by various factors such as, for example, the inconstancy of the speed at which the recording head ejects the ink and the inconstancy of the speed of motion of the carriage. It is not easy to control the timing for discharging the ink during the reciprocating printing because the factors mentioned above are not constant in magnitude relative to the direction of the advance of the carriage. Besides, since the distance from the head to the paper surface in the recording apparatus is dispersed appreciably among the individual apparatuses shipped from the production plant, the control of the landing positions of ink dots in the forward and the backward passes due to the adjustment of the timing for discharging the ink has its own limit.
In the conventional recording method which uses such a multi-head as described above, the timing (frequency) for continuous discharge of ink through the individual nozzles is determined by the density of picture elements in the recorded image and the speed of motion of the carriage. If this timing cannot be controlled with amply high accuracy, the ink dots for recording on the surface of the paper as the recording medium are incorrectly arrayed relative to the scanning direction of the carriage and the multi-heads, with the result that the recorded image will betray uneven density and inferior quality. As a result, the recording ink dots produced by the head are allowed to form an ideal image array only when the throughput is exalted to the fullest possible extent and the head is driven under conditions such that the limit of frequency of the head and the given density of picture elements may be simultaneously satisfied with high accuracy.
Incidentally, in the conventional test print, the method of printing vertical linear patterns perpendicular to the direction of scan as spaced at an interval of not less than several mm is generally adopted for the sake of the test print pattern itself intended to select the optimum conditions and for the purpose of enabling the operator to make his decision as to the selection. FIGS. 4A and 4B depict such vertical linear patterns.
One example of the conventional method for adjusting a reciprocating registration is illustrated in FIGS. 36A to 36F. FIGS. 36A (1) and 36A (2) respectively represent forward print data and backward print data for carrying out the reciprocating printing of the type allowing the feeding of a recording medium to intervene between the passes in the two directions. The vertical lines perpendicular to the direction of reciprocating scan which are illustrated in FIG. 36D constitute themselves the record pattern which is obtained by adjustment of normal registration based on the data mentioned above. Specifically, one vertical rectilinear test pattern is formed by printing vertical straight lines of 8 dots as spaced at a lateral interval of 4 dots in the forward and the backward passes. Heretofore, it has been customary that when the print timing in the two directions shows a deviation of 1 pixel or more in any of the vertical rectilinear test patterns, this deviation permits the operator to determine whether or not the particular test pattern is dispossessed of its rectilinearity and, after completion of the printing, select from among the plurality of test patterns one possessed of the best rectilinearity and insert the relevant numerical data of the choice test pattern somehow in the recording apparatus proper.
It has been also customary heretofore that the head is caused by a certain existent condition of itself to be moved along the longitudinal axis and, when this motion is made, the operator is enabled to insert the relevant numerical data of this motion into the recording apparatus proper and adjust the subsequent print timing during the reciprocating printing to the correction value.
The rectilinearity of the vertical rectilinear test patterns mentioned above, however, is such that any deviation exceeding 1 pixel can be visually discerned, while a deviation smaller than 1 pixel is not easily discerned visually. FIGS. 36B, 36C, 36E, and 36F represent the test patterns which aptly permit the visual determination of rectilinearity. They are record patterns for determining correction values for the compensation of positional deviation. They are obtained by successively varying the timing of backward print at an increment of 0.25 pixel from the record patterns of FIG. 36D as the median. Heretofore, it has been customary that the test patterns of FIGS. 36B, 36C, 36E, and 36F are rated as substantially equaling those of FIG. 36D. The criterion heretofore adopted for visually rating and adjusting the test print is the unit of at least 1 pixel.
Particularly when the reciprocating printing is performed by reciprocating the recording head relative to the recording width of the recording head while the recording medium is kept in a suspended state or when a plurality of color heads are parallelly driven, the maintenance of the optimum image quality by the control with a fixed drive parameter is likely to encounter an obstacle, such as by changes in the circumstance in which the printer is being used.
Specifically, the adversity in question will be briefed below with reference to FIGS. 60A, 60B and 61. FIGS. 60A and 60B illustrate the manner in which a head 901 fixed on a carriage 706 in motion at a speed S ejects an ink drop at an angle .theta. and a velocity V onto a paper surface placed at a distance P from the head respectively in the forward pass (FIG. 60A) and the backward pass (FIG. 60B). The carriage speed is S in the forward pass and -S conversely in the backward pass and the angle of ejecting is fixed constantly at .theta.. Now, let .DELTA.A and .DELTA.B stand for the distance from the position at which the head ejects the ink drop to the position at which the ink drop lands on the paper surface in the forward and the backward pass, respectively, relative to the direction of scan, then they will be expressed by the following formulas. EQU .DELTA.F=P.times.(V sin .theta.+S)/V cos .theta. EQU .DELTA.B=P.times.(V sin .theta.-S)/V cos .theta.
Thus, the timing of ink discharge relative to a target picture element in the forward and the backward pass differs in terms of distance as follows. EQU (.DELTA.F-.DELTA.B)=P.times.2S/V cos .theta.
If this magnitude is invariably constant in all the recording apparatus and recording heads, the positions of ink dots in both directions will be kept duly corrected by driving the head constantly at a timing fit for ideal ink discharge. Actually, however, there exists the possibility that the thickness of a recording paper varies P, the uneven carriage speed varies S, and the inconstancy of craftsmanship among recording heads varies the speed of discharge V. Even one and the same head possibly imparts a variation to the speed of ejecting the ink owing to such factors as temperature and direction of scanning or eventually induces gradual variation in this speed owing to the effect of protracted of use.
FIG. 61 shows the magnitudes of the distances .DELTA.F and .DELTA.B, the difference (.DELTA.F-.DELTA.B), and the amount of positional deviation of ink dots which are found when the distance P, the carriage speed S, the ejection speed V, and the discharge angle .theta. shown in FIGS. 60A and 60B are varied in the forward and the backward pass.
The data given in the uppermost row of the table of FIG. 61 represent the case of using the paper distance P=1.2 mm, the carriage speed S=4.318 m/sec (equal in forward and backward passes), and the ejection angle .theta.=10.degree. (equal in forward and backward passes) as conditions and reporting "0" as the magnitude of positional deviation of ink dots in the two directions on the assumption that the head is driven so as to satisfy the magnitudes of .DELTA.F and .DELTA.B and the optimum correction value (.DELTA.F-.DELTA.B)=84.18 .mu.m.
In contrast, the data given in the second and following rows of the table show the fact that the proper correction value (.DELTA.F-.DELTA.B) is varied from one case to another because the magnitudes of various factors are varied little by little. Since the head is driven in these cases with the same timing as that used for the operation yielding the data of the uppermost row, varying amounts of positional deviation of dots inevitably arise. Thus, the magnitudes of deviation indicated in these rows represent differences of optimum correction value (.DELTA.F-.DELTA.B) respectively from the magnitude shown in the uppermost row.
In the table of FIG. 61, the individual factor values are varied within the ranges generally accepted for variation of such values. It is remarked from this table that the factor capable of affecting the deviation of dots in the two directions to the greatest extent is the paper distance P. From the table, it is noted that when the paper distance fixed at 1.2 mm is varied by a correction value of only .+-.0.2 mm, this variation gives rise to a deviation of 42.29 .mu.m (not less than a half picture element at a picture element density of 360 dpi). The recording papers of an ordinary run have a staple thickness of about 100 .mu.m. The variation of thickness of the order just mentioned is easily affected by the inconstancy of the paper distance among the recording apparatuses proper and the inconstancy of craftsmanship among the recording heads. Thus, the corrections must be made in accordance with the conditions of a particular recording apparatus.
The variation of the paper distance under discussion which is caused by the inconstancy of the recording apparatus itself can occur while the printing is in process. Ideally, the part of the recording paper engaging in the printing operation should be kept in a flat smooth state by paper retainers disposed one each before and after the site of printing. When the printing duty is high or when the split recording method which completes a print by dividing one and the same recording scan into a plurality of recording scans is adopted, however, the part of the recording paper already used for printing possibly entails shrinkage of fibers therein because of the absorption of the ink. Thus, this particular part is likely to be raised from the normal level. In this case, the paper distance P is apt to vary in the forward and the backward pass in each recording scan. This rise of the paper (hereinafter referred to as "cockling") varies the optimum correction value and consequently gives rise to a positional deviation of dots during the reciprocating printing.
As described above, the correction value can not be kept constant because of various factors. It has been ascertained that the correction of the positions of ink dots is desirable when the reciprocating printing is carried out or when the record is produced with a plurality of heads.
In the case of reciprocating printing or in the case of producing a record by use of a plurality of heads, since such dot positions are evaluated and corrected based on the rectilinearity of vertilines of a specific pattern as described above, the accuracy of determination is inevitably limited and the maintenance of ideal image quality is likewise limited. In short, the conventional method which relies for determination of dot positions on such vertical lines as described above does not easily permit correct determination for the sake of such fine adjustment as involves a microscopic size of several .mu.m or not more than one picture element.
Besides, the recent exaction of the manufacture of printed images of improved quality at a great speed has reached a point where a novel method which allows correction of dot positions and, at the same time, permits compensation with such a minute unit as mentioned above serves as an indispensable tool. The conventional printer, on encountering a change in the material or thickness of the recording medium, requires a varying measure to counter the change and obtains an ideal image condition with difficulty. Thus, the desirability of infallibly and accurately accomplishing ideal recording characteristics without being affected by such factors of the recording medium as material and thickness has been gaining in significance.
A system which allows the operator of a printer to evaluate the test print image easily and accurately remains yet to be developed.